Ophthalmology: Sub-specialty—Corneal, refractive and cataract surgery

Corneal and laser refractive surgery

This is a very exciting area of ophthalmology. With increasing numbers of people having laser refractive surgery, they have to understand the consequences of this.

Aims

• Surgical management of keratoconus.
• Principles of corneal surgery.
• Know about current laser refractive techniques.

Cornea

The healthy cornea is clear, tough and free of blood vessels or opacification and acts as an important refractive surface. Corneal clarity is dependent on the integrity and normal functioning of all five of its cellular layers. Of particular importance is the corneal endothelium, a single cell layer of non-mitotic cells that maintains the cornea in a state of partial ‘dehydration’ and hence transparency through the use of cellular metabolic ‘pumps’.
When that pump fails, the stroma swells and becomes hazy, for example in bullous keratopathy requiring a bandage contact lens (CL) or surgery (penetrating or lamellar).
Keratitis causing corneal scarring can require PK. In order for the graft to take, the host cornea should not have many blood vessels, which could contribute to corneal graft rejection, and it should have near or near-normal sensation. A partial-thickness corneal graft is a lamellar keratoplasty.

Corneal diseases

Keratoconus

Keratoconus is the most common primary ectasia of the cornea. It is characterized by progressive thinning of the cornea, which takes on a cone formation and leads to irregular myopic astigmatism. It is a chronic progressive dystrophy that commonly starts in the second decade of life. This conical misshaping leads to astigmatism, which is variable or ‘irregular’ across the surface of the cornea. It is important to remember that most refraction of light occurs at the air–cornea interface rather than at the lens inside the eye, so significant changes to the cornea have a major impact on vision. Normal or ‘regular’ astigmatism can be corrected by glasses, but irregular astigmatism cannot. Contact lenses are usually required to make the air–cornea interface more spherical to allow the appropriate bending of the light and focusing of the eye. Early stages of the disease are usually asymptomatic and may be picked up on corneal topography, a scan which maps the corneal contours. This may be found in patients seeking corneal refractive surgery where a preoperative corneal topography scan is mandatory. Patients with keratoconus, even if it is asymptomatic, should not have LASIK as this treatment thins the cornea further and makes the keratoconus worse.

Signs

• Usually bilateral but asymmetrical.
• Vogt striae: fine vertical lines in the centre of the cornea.
• Fleischer ring: epithelial deposits of iron around the base of the cone.
• ‘Oil droplet’ reflex on direct ophthalmoscopy.
• ‘Scissors’ reflex on retinoscopy.
• Munson sign: bulging of the lower eyelid on downgaze. This is usually seen only in advanced disease, where the cone shape of the cornea pushes against the lower lid when the eye moves into downgaze.
• Stromal scarring: this is usually seen when there has been a rupture of Descemet’s membrane, the membrane just outside the endothelial layer of the cornea. When this occurs, the cornea becomes painful and oedematous as the pump function of the endothelial cells is disrupted.

Associations

• Down syndrome
• Ehlers–Danlos syndrome
• Marfan’s syndrome
• osteogenesis imperfecta
• general atopy and vernal keratoconjunctivitis.

Management options

• Contact lenses.
• Collagen crosslinking; this technique uses riboflavin drops on the eye, which is then exposed to ultraviolet A light. It can stabilize or prevent progression of early forms of the condition.
• Corneal ring segment implants (e.g. Intacs®). These implants are placed into pre-cut tunnels within the stroma of the cornea to alter its shape.
• Corneal graft. This may be a full-thickness graft known as PK, or a lamellar or layered graft known as a deep anterior lamellar keratoplasty (DALK).

Pterygium

Microscopic lamella excision is needed if it encroaches on the visual axis and corneal neoplasia.

Corneal surgery

Corneal grafting

From human donor material. Donor eyes are stored in eye banks (e.g.
in London, Bristol, Manchester and Dublin). Donors are screened as
all human tissue transplant material is.

• PK (Figure 35.2): a full-thickness transplant of all five layers of the cornea. This can be used for corneal dystrophies or perforations.
• DALK: replacing the anterior two-thirds of the cornea only. The recipient retains his or her own corneal endothelium, Descemet’s layer and some very deep stroma. This transplant is suitable for conditions that affect the anterior portion of the cornea only (e.g. keratoconus or a corneal scar following infection). The advantages of this graft include less rejection risk and the absence of a full-thickness wound in the cornea, which can separate if the grafted eye is subjected to trauma postoperatively. The main disadvantage is that there will always be an interface between the donor anterior portion of the cornea and the recipient’s retained posterior portion of the cornea, and this can impact vision. It is more difficult to perform than PK.
• Endothelial keratoplasty (Descemet’s stripping endothelial keratoplasty (DSEK) or Descemet’s stripping automated endothelial keratoplasty (DSAEK)): this graft is used in patients whose endothelial layer of pumps has failed and who have consequent corneal cloudiness.
  Only the inner layer of the cornea is transplanted.

Refractive surgery

Surgery and/or laser is used to reshape the main refractive surface of the eye (the anterior corneal surface), and to bring light rays in focus on the retina without the need for glasses or contact lenses.
The main indication is myopia.
There are three methods of laser, in all of which the excimer laser is used to correct myopia, hypermetropia and astigmatism under local anaesthesia.

In myopia, the corneal surface is flattened so that the image focuses onto the retina. The effect in hypermetropia is not always stable.

Photorefractive keratectomy (PRK)

Excimer laser ablation reshapes the anterior corneal surface after manual debridement of the epithelium.

• Suitable for low refractive errors (−1 to −6 D).
• Disappointing for hypermetropia.
• Bandage contact lenses as eye painful for 48 h.
• Variable slow healing and remodelling over months.
• Quality of vision—temporary haze common after treatment.
• Regression can occur in higher degrees of myopia (>−10D).
• Recurrent erosions are a complication.

Laser surgery

Laser-assisted intrastromal in situ keratomileusis (LASIK)

A partial-thickness corneal-hinged epithelial ‘cap’ is raised, and the excimer laser ablation is applied to stromal tissue below this, after which the ‘cap’ is replaced.

• Suitable for higher degrees of myopia with less risk of regression (up to −10D).
• Disappointing for hypermetropia.
• No haze.
• Best visual acuity achieved more rapidly than with PRK.
• Dry eye for a few months.

Laser epithelium keratomileusis (LASEK)

This can be used for low myopia and hypermetropia and in patients who are unsuitable for LASIK. The epithelium is treated with alcohol to loosen it. It is preserved with the intention of replacing it when the stroma has been ablated with the excimer laser.

Femtosecond-assisted LASIK

This differs from LASIK in that the flap is ‘cut’ using a laser rather than a blade known as a microkeratome.
The proposed advantages include more accurate thickness of the flap and lower probability of incomplete flap or button-hole of the flap.

Blended vision

Laser blended vision, is a sophisticated laser eye treatment which rectifies short-sightedness (myopia) in one eye and long-sightedness (hyperopia) in the other eye. Primarily the treatment is for presbyopia, the progressive loss of the ability to focus on nearby objects around aged 45–50 years.
Blended vision can be achieved through laser eye surgery such as LASIK, PRK or LASEK, by correcting the dominant eye and the shape of the cornea, modifying it for reading vision and still maintaining good distance vision. In other words, one eye is focused for distance and the other eye is focused for near vision. The eyes are effectively working together to allow good vision at near, intermediate and far, without needing glasses. The effects of blended vision treatment can last between 5 to 10 years.

Who can have laser refractive surgery?

• Patients should be >21 years old, have a stable refraction, not be pregnant and have no keratoconus, cataract or glaucoma.
• They should not be on systemic steroids.
• Patients should have realistic expectations and have been warned of potential side effects of haze, night-time glare, ghosting images, starburst around lights, dry eye and risk of macular haemorrhage.
• It is also essential that patients are warned that intraocular pressure and A-scan and biometry measurements cannot be made accurately after refractive surgery.

Cataract assessment

Cataract causes gradual loss of vision in a white eye in older persons.
Learn to assess if a patient has a cataract using a direct ophthalmoscope.

Aims

• Appreciate the gross anatomy of the crystalline lens.
• Understand there are important causes that can accelerate the formation
  of cataracts.
• Know the factors considered before surgery.
  The term ‘cataract’ is derived from the Latin word for waterfall, ‘cataracta’, as it is believed that the appearance of pathological lens opacities is akin to that of the magnificent waterfalls seen in nature. Other than refractive error, cataract is the most common cause of visual morbidity in adults worldwide. We are fortunate in the United Kingdom, where treatment can be given quickly and efficiently once symptoms are too troublesome for patients. Currently, cataract surgery is the most commonly performed ophthalmic surgical procedure in the United Kingdom.

Anatomy

• Think of the human lens as a peanut M&M held in position at the sides by the zonules extending from the ciliary body. The outer crystal coating is analogous to the lens capsule, the chocolate is equivalent to the lens cortex and the peanut is the nucleus!
• Surgically, we delicately tear a hole in the front part of the lens capsule in order to access the deeper parts of the lens: the idea is to leave the remaining lens capsule in place as a bag to support the implanted plastic intraocular lens (IOL).
• Cataracts are classified depending on the part of the lens that’s opacified:
  ○ cortical
  ○ posterior subcapsular
  ○ nuclear sclerotic.

Causes of cataract

• Old age (commonest).
• Associated with other ocular and systemic diseases.
• Associated with systemic medication (steroids and phenothiazines),
  discussed further on in this chapter.
• Trauma and intraocular foreign bodies.
• Ionizing radiation (X-ray and UV).
• Congenital (dominant, sporadic or part of a syndrome, abnormal galactose metabolism or hypoglycaemia).
• Associated with inherited abnormality (myotonic dystrophy, Marfan’s syndrome, Lowe’s syndrome, rubella and high myopia).

Risk factors

All individuals will develop a cataract if they live long enough.
However, certain circumstances accelerate the development of a cataract.
It is important that in any referral letter these issues are highlighted, so the surgeon can be prepared pre-operatively (e.g. previous trauma may suggest weak zonules).

• Systemic causes:
  ○ diabetes mellitus
  ○ myotonic dystrophy
  ○ Wilson’s disease.
• Ocular causes:
  ○ inflammatory eye disease
  ○ previous ocular surgery.
• Trauma
• Congenital conditions:
  ○ metabolic disorders.
• Drug history:
  ○ steroids
  ○ amioderone
  ○ phenothiazines.

Natural history of cataract

Typically, the nuclear sclerotic cataracts increase the refractive power of the eye, so-called refractive myopia. Hence, initially optometrists in the community can manage cataracts by prescribing greater minus lenses. If there are no other ophthalmic morbidities, these patients do not need a referral.
At this stage, examination with the direct ophthalmoscope in General Practice or Accident & Emergency will reveal an abnormally dim red reflex. Nuclear cataract causes a central black shadow, whilst cortical cataracts cause black spoke-like shadows coming from the edge of the red reflex. These spokes are also seen in the visually insignificant, congenital opacity termed a ‘blue-dot cataract’ (Figure 36.2).
There will come a time when spectacles alone cannot correct the refractive error: it is at this time that patients should be referred to the eye clinic for cataract surgery. If patients delay presentation, the cataracts will become dense, mature and brunescent (Figure 36.3).
Surgery should be undertaken in the following situations:

• Symptoms of:
  ○ reduced best-corrected visual acuity (BCVA) less than 6/9
  ○ monocular diplopia
  ○ Glare in sunshine or with street or car lights (particularly with dense posterior subcapsular cataracts across the visual axis).
  ○ Clouding of vision (particularly with cortical lens opacities).
• Patients with diabetes mellitus
  ○ otherwise, retinopathy changes cannot be monitored.
• Treating phacomorphic glaucoma
  ○ A large, swollen lens can physically cause the iris to bow forward.
  This results in a narrowing of the irido-corneal angle and a subsequent rise in the intraocular pressure (IOP) due to poor aqueous outflow.
• Patients with poor BCVA due to other causes:
  ○ All patients undergoing surgery should have dilated fundoscopy and optical coherence tomography examinations to look for causes of reduced BCVA other than cataracts, such as macular scarring secondary to age-related macular degeneration. In this scenario, although vision cannot be improved, surgery may be considered in order for patients to have better colour contrast.
• Preventing amblyopia:
  ○ A cataract in children will cause stimulus deprivation amblyopia.

Preparation for cataract surgery

Each member of the team needs to play an active role for surgery to be successful:

• Biometry (Figure 36.4):
  ○ Arguably, this is the most important part of surgery. Usually, the nursing staff perform an ultrasound measurement of the length of the eye and keratometry to measure the curvature of the cornea. This enables the surgeon to decide which strength of lens should be implanted and where on the cornea the incision should be made.
• Medical pre-assessment:
  ○ Warfarin should not be stopped, but the international normalized ratio (INR) should be within the therapeutic range.
  ○ One must know if the patient is on tamsulosin, as this causes floppy iris syndrome and makes the surgery more technically difficult.
  Tamsulosin is the only alpha-receptor antagonist to have this effect.
  ○ Confirm that general medical conditions are stable, particularly hypertension, respiratory disease and diabetes. Remember that this is elective surgery and unless a patient can lie supine for 20 minutes, surgery should be reconsidered.
• Informed consent:
  ○ The operating surgeon should inform patients of expected outcome and the complications of surgery.
  ○ The surgeon should take great lengths in discussing the theatre environment and procedure steps so the patient does not feel apprehensive throughout. Remember that the operation will occur whilst the patient is awake, and this can be very unnerving. So take time to explain that:
  ■ There will be a drape over their head, but oxygen will be supplemented underneath.
  ■ They will hear talking amongst the staff in order to carry out the procedure safely.
• ■There will be various members of the team in the theatre, including nurses, operating department practitioners, anaesthetists, junior doctors and medical students.
• ■ During parts of the operation, they will hear buzzing sounds. Furthermore, they will receive constant irrigation in the eye, and sometimes this may trickle down into their ear.
• ■ If they wish to move or cough, to let you know in advance to enable the safe removal of instruments outside the eye before they do so.

Cataract surgery

Aims

• Understand the methods of anaesthesia.
• Understand the principles of cataract removal.
• Be able to explain cataract surgery to a patient.
  Microsurgery in developed countries has reached a very high standard due to the developments in microsurgical instruments and intraocular lens design. There is controlled and precise removal of the lens with the assistance of an operating microscope. The majority of the lens capsule is retained to hold the implant within it. An operating microscope is needed. In order to reach the lens, a small corneal incision is made close to the limbus for the phacoemulsification (phaco) probe.
  It is important to appreciate the anterior chamber depth and to keep all instruments away from the corneal endothelium, in the plane of the iris (Figure 37.1).

Definitions

• Phakia: an eye with a natural lens in situ.
• Pseudophakia: an eye that has had a cataract removed and an artificial intraocular lens implanted (Figure 37.2).
• Aphakia: an eye that has had a cataract removed without an artificial lens inserted.

Anaesthesia for cataract surgery

Local anaesthesia

• Topical: drops of proxymethocaine; there is full kinesis of the extraocular muscles (EOM), and hence you need a compliant patient and a confident surgeon.
• Subtenons: dissect through the conjunctiva and Tenon’s capsule, inferomedially. Use a blunt cannula to administer 2 ml of lidocaine within this plane.
• Peribulbar injection: a 21-gauge needle is used to distribute 5–10 ml lidocaine within the orbit. It can be administered via the skin or through the conjunctiva.
• Retrobulbar injection: direct 1–2 ml lidocaine within the muscle cone. This is now becoming less commonly used, due to the risk of retro-bulbar haemorrhage.

Sedation

Intravenous drugs may be given with local anaesthetics but are not preferred as the patient could drift off to sleep and then suddenly wake up with a jolt and move his or her head—which is undesirable in cataract surgery.

General anaesthesia

This is used for young and uncooperative patients.

Surgical technique for cataract removal

• Patients have to lie supine so a microscope with a bright light and good magnification can be positioned above them.
• The surgeon works from the side or above the head, looking down the microscope, using the red reflex from the retina to aid cataract removal.
• Operating superiorly, the surgeon has the advantage of support for his or her hands from the patients’ brow. However, deeper set eyes are difficult to access because of this.
• Temporally, the surgeon has the advantage of less restriction of hand movements. Conversely, he or she lacks support for stabilization.
• The incision should be made dependent on the keratometry result of the patient. Operate on the steeper axis as the incision will flatten this axis and steepen the cornea 90° to this: this phenomenon is called ‘coupling’. Both effects act to reduce the astigmatic error.

Draping

Before surgery can start, the eyelids and lashes are covered with a thin transparent plastic drape in order to keep contaminated lashes out of the surgical field. Staphylococci live in abundance on the lashes. The drape is light and also covers the face—lifted up from it as a small tent—to protect the face from irrigation fluids used in the surgery, which are collected into a small bag at the side of the head.

Small speculum

The eyelids are kept open by a combination of the drape and a small wire speculum, which does not cause the patient discomfort.

Surgery

• Extracapsular cataract surgery is very rarely used in the United Kingdom now. The procedure involves removal of the entire nucleus as one piece; the soft cortex is aspirated, and a rigid or soft implant is inserted. The corneal wound is large and requires sutures to close, which are removed as late as 8 weeks after surgery.
• Small incision sutureless cataract surgery (SISCS) is not used in the United Kingdom but is commonly used in places like India. The surgery involves creating a long scleral tunnel and expressing the nucleus as a whole. A phaco machine is not required in this technique.
• Phacoemulsification and intraocular lenses (IOLs) comprise the technique of choice in the United Kingdom, and trainees are taught this from the ST1 level. The steps are highlighted on the diagrams, but briefly:
  ○ The main incision is made by a keratome and is about 3 mm in size (small incision).
  ○ A small needle (cystotome) is used to create a circular hole in the front part of the lenticular capsule.
  ○ A cannula is used to inject water in the plane between the lens and capsule (a process called hydrodissection).
  ○ The lens is cracked into small fragments using a sharp chopper and the main phaco probe. Techniques used commonly are divide and conquer, stop and chop and vertical chop.
  ○ Fragments are irrigated through the probe, followed by the soft lens matter adherent to the inner capsule.
  ○ A soft, foldable IOL implant can be inserted through the small incision into the remaining lens capsule (posterior chamber IOL).
  This incision is usually sutureless.

Implant power

The IOL power is carefully calculated to take into account the patient’s postoperative visual requirements. After surgery, the lens is unable to accommodate, so the patient and surgeon decide preoperatively what type of vision the surgery should aim for. Most commonly, patients choose to be left emmetropic for distance and need glasses to read.
Emphasize that whilst all efforts are made to achieve this aim, the surgeon cannot guarantee a particular result and the patient may require glasses to also refine distance vision.
Multifocal IOLs (for both near and far correction) are available but not commonly used. Toric lenses are used more readily, in order to correct astigmatism that is too great to be corrected solely by placing the primary incision along the steep axis. In practice, steepening of greater than 2 dioptres merits thought of using a toric lens.

Cataract surgery postoperative care

Although cataract surgery is technically highly specialized, there still can be complications which are sight threatening.

Aims

• Be aware of normal and undesirable postoperative events.
• Recognize postoperative complications from history.

Routine postoperative management

Patients are discharged as day cases and are usually given a clear shield to wear over the operated eye. Steroids and antibiotic drops are administered four times daily for 2–4 weeks after surgery. During that time, patients can read, take gentle exercise, shop, shower or bath and wash their hair carefully. Spectacles can be prescribed from 6 weeks after surgery.

Undesirable postoperative events (complications)

Watering and a foreign body sensation are common after surgery (Table 38.1).
As with any surgical complications, think about them in time order:

• Acute:
  ○ Endophthalmitis (Table 38.2):
  ■ an intraocular infection of the whole eye
  ■ This is the most important sight-threatening complication following surgery and requires urgent admission and treatment.
  ■ Its onset is usually 4–5 days after surgery. The most common organism causing endophthalmitis is coagulase-negative Staphylococcus, particularly S. epidermidis.
  ■ The signs and symptoms that one should look for in such patients are:
• severe pain
• loss of vision
• injected conjunctiva
• hypopyon (Figure 38.1)
• intense inflammation and fibrin plaque formation (Figure
  38.2)
• loss of red reflex.
• Subacute:
  ○ Postoperative cystoid macular oedema (CMO) or Irvine–Gass syndrome (Figure 38.3 and Table 38.2);
  ■ Classically, this occurs at 6 weeks postoperatively.
  ■ The vision, which had hitherto been clear following surgery, suddenly drops, and the patient complains of distortion.
  ■ Treatment involves topical steroids, non-steroidal antiinflammatory drugs and carbonic anhydrase inhibitors (CAIs).
• Chronic (months to years following surgery):
  ○ Posterior capsular opacification (PCO) (Table 38.2);
  ■ The bag containing the lens implant (the remaining part of the capsule) over time can accumulate mucopolysaccharides. Certain intraocular lenses have a greater tendency to do this, and the risk is increased if strands of soft lens matter are left in situ during surgery.
  ■ Patients describe a frosted appearance of their vision and have symptoms similar to their original cataract formation (a so-called after-cataract).
  ■ Treatment is a simple yttrium–aluminium–garnet capsulotomy laser procedure (Figure 38.4).
  ○ Rhegmatogenous retinal detachment (RRD);
  ■ 1.5% of patients undergoing surgery develop retinal detachment.
  This is particularly pertinent if the posterior capsule (PC) was breached inadvertently during surgery and vitreous spilled over into the anterior chamber (a so-called PC tear).
  ○ Bullous keratopathy;
  ■ This is associated with complicated surgery.
  ■ The surgical field that ophthalmologists operate in is about 3 mm from the endothelial cells of the cornea to the anterior capsule of the lens. If the surgeon is not extremely careful, some of the endothelium can be scraped off during surgery.
  ■ The endothelium contains pumps to continually remove fluid back out to the AC.
  ■ If enough cells are scraped off, eventually the cornea decompensates, becomes cloudy and develops painful bullae.